Started a batch of Carmel Hard Apple Cider on Wed. noticed this morning that liquid level in my airlock has gone down.
I've heard of cider back sucking before but what causes this and is it a bad thing?

If the temperature of the juice cools before fermentation starts, that can do it. You want to avoid this, as airlock water water can get funky, but since it already happened and you are only a couple days in, I wouldnt worry about it too much.

Started a batch of Carmel Hard Apple Cider on Wed. noticed this morning that liquid level in my airlock has gone down.
I've heard of cider back sucking before but what causes this and is it a bad thing?

Change in temp usually causes this. Did the cider cool down after you put on the air lock?

It's also happened to me when I put the air lock on a bucket, then picked the bucket up and moved it somewhere else.

I had the same problem yesterday. I use a 6 gallon glass carboy so I know it wasn't the squeezing that caused the drop in levels in the airlock. I guess it was just maybe the cooling that was taking place. This time I used vodka as the liquid instead of water. It is slowly bubbling away today so things are good.